vdpa/mlx5: Restore cur_num_vqs in case of failure in change_num_qps()

Restore ndev->cur_num_vqs to the original value in case change_num_qps()
fails.

Fixes: 52893733

 ("vdpa/mlx5: Add multiqueue support")
Reviewed-by: Si-Wei <Liu&lt;si-wei.liu@oracle.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20220105114646.577224-10-elic@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>